This application is related to U.S. patent application Ser. No. 10/716,808, the entire contents of which are incorporated herein by reference.
Embodiments of the invention relate to improving NEXT, FEXT, balance (TCl, TCTL) and return loss in terms of magnitude and upper frequency limits of transmission components and products. Embodiments of the invention are techniques used to apply compensation at the point of the NEXT noise sources. Standard compensating techniques involving inductive and capacitive coupling to cancel crosstalk (NEXT or FEXT) can only achieve limited success due to the limits defined by the TIA and IEC in terms of the magnitude and phase for both NEXT and FEXT of qualification plugs. In addition, standard compensation techniques are usually applied away from the outlet/plug interface which is the major contributor to crosstalk. As performance requirements are pushed beyond 100 and 250 MHz up to and above 500 MHz, canceling the crosstalk at the source becomes more critical. ANSI/TIA/EIA-568-B.2-1 is represented in Table 1 below defines the magnitude and phase requirements for category 6 test plugs. As an example, the TIA specifies the case 1 plug to have a specified magnitude (36.4 at 100 MHz) and phase −90±1.5*f/100. The component/connector design (outlet/PCB and cable termination area) must cancel this. In essence it must have the same magnitude of NEXT, but opposite phase, +90 degrees as shown in FIG. 1.
In reality it is difficult to match this phase perfectly. To match perfectly, one can imagine folding the plot in FIG. 1 in half, the +90 and −90 lines would fall on each other and cancel each other out.
In a perfect match, where magnitude and phase are equivalent and applied at the point of the NEXT source, the resulting NEXT is at the noise floor, virtually nonexistent. FIG. 2 shows the plug/outlet interface 10 and the PCB/cable termination area 12. A plug 14 is mated with an outlet 16 mounted to PCB 18. A connecting block 20 is mounted to the PCB 18 as known in the art. Signals travel along a path including plug 14, outlet 16, PCB 18 and connecting block 20 at which cables are terminated. If we consider the outlet/plug area 10 as the primary offending crosstalk contributor, the PCB/cable termination area 18 typically compensates for this offending crosstalk.
In most existing design implementations, the compensating crosstalk cannot be added/applied at the point of origin, that is the plug/outlet interface 10. Typically, compensating crosstalk is added on the PCB and cable termination area 12. Unfortunately, it is difficult, if not impossible, to replicate the exact magnitude and phase of the offending crosstalk throughout the frequency range. There is a phase shift due to the distance from the plug/outlet interface 10 to where the compensating crosstalk is applied. It should be noted, the geometry and location of the outlet contacts that go from the outlet/plug interface 10 to the PCB (or outlet to connecting block in a lead frame design) may affect the magnitude and phase of the offending crosstalk. Therefore, the PCB 18, cable termination area and connecting block 20 or other termination must compensate for what remains. It must also compensate when tested in both directions. In addition, the TIA and IEC specify a range of performance for the modular plugs which directly contribute to the offending crosstalk.
If we assume the magnitudes are equal for the crosstalk at the plug/outlet interface and the compensation on the PCB/cable termination area, but out of phase, we get a non-category 6 compliant response as shown in the 0 picosecond delay plot of FIG. 3. By shifting the phase via manipulating the delay as shown in FIG. 4, we can extend the upper frequency performance of the design without manipulating the magnitude. The key is to create the null at an area where it pulls the performance down, below the specified limit line. FIGS. 3 and 4 show the affect of varying the delay in 20 ps intervals. The null occurs at a frequency where magnitude and phase are equal.